activator: add RFC-18 flex-algo CHANGELOG entry

Author: ben-malbeclabs

CHANGELOG.md

@@ -43,6 +43,16 @@ All notable changes to this project will be documented in this file.
   - Add `doublezero-admin migrate flex-algo [--dry-run]` to tag existing links with UNICAST-DEFAULT and backfill node segments
 - SDK
   - Update Go, Python, and TypeScript SDKs with `TopologyInfo` deserialization and new `link_topologies`, `link_flags`, and `include_topologies` fields
+- Controller
+  - Add `features.yaml` config file support (`--features-config` flag); a `flex_algo.enabled` flag gates all flex-algo config generation
+  - When flex-algo is enabled: generate IS-IS TE admin-group assignments on link interfaces, flex-algo topology definitions, and `system-colored-tunnel-rib` as the BGP next-hop resolution source
+  - Stamp BGP color extcommunity on user tunnel route-maps for tenants with `include_topologies` set
+  - Add `node-segment ipv4 index` lines to Vpnv4 loopback config for each flex-algo, sourced from segment routing IDs backfilled by the activator
+- Activator
+  - Automatically backfill flex-algo node segment IDs for all activated devices when a new `TopologyInfo` account is created onchain
+  - Automatically backfill existing topologies' node segments when a Vpnv4 loopback interface is activated on a device
+- E2E Tests
+  - Add flex-algo e2e tests: topology lifecycle (safe-delete enforcement), topology filter (`--topology` CLI validation), unicast-drained flag transitions, tenant `include_topologies` changes, migration dry-run and live run, and controller config generation against live cEOS
 
 ## [v0.15.0](https://github.com/malbeclabs/doublezero/compare/client/v0.14.0...client/v0.15.0) - 2026-03-27